As used herein, refractory fibers are defined as inorganic, amorphous, or crystalline fibers which are not adversely affected by temperatures in excess of 1500.degree. F. A typical refractory fiber is a glassy fiber melted from equal amounts of pure oxides of Al.sub.2 O.sub.3 and SiO.sub.2. The melt is fiberized by techniques which are well known in the art. This typical fiber is heated at 2300.degree. F.-2400.degree. F. and exhibits shrinkage of 3-4% in 24 hrs. exposure at these temperatures.
Another class of fibers is made by melting kaolin clay. A good discussion of such fiber is found in Inorganic Fibers, by C. Z. Carroll-Porczynski, published in 1958 by Academic Press Publishers in New York. At pages 162-164 of this reference a Kaowool.RTM. fiber is disclosed as being marketed by Babcock and Wilcox. Due to impurities of the commercial grade kaolin used, this fiber exhibits linear shrinkage of 5-8% at 2300.degree. F.-2400.degree. F. In addition kaolin fibers generally contain a high percentage of shot which is nonfiberized material. Shot may deleteriously affect the thermal insulating properties of the refractory fiber. The greatest advantage of kaolin based fiber is that the cost of kaolin is very low compared to the pure oxides utilized in melting the fiber discussed below.
It has been disclosed that the addition of zirconia to pure oxides of alumina and silica will improve the melt rate and fiberization of refractory fibers. U.S. Pat. No. 2,873,197 issued to J. C. McMullen on Feb. 10, 1959 contains such disclosure. While improving the fiber yields and process, the addition of zirconia does not allow fiber to be made as easily as kaolin based fibers.
A previous application of which the present inventors were aware, discloses a fiber made by melting pure oxides of Al.sub.2 O.sub.3, SiO.sub.2 and ZrO.sub.2. As with the fibers disclosed in McMullen, these fibers melt more rapidly than pure alumina-silica fibers. However, Applicants disclose therein that unexpectedly low linear shrinkage was found for fibers melted in a very narrow combination percentages of pure alumina-silica-zirconia. In another range, it was found that the addition of zirconia imparted acid and alkaline resistance to the fibers. The disadvantage of fibers made from the disclosed pure oxides is that the fiber is made from a three component system utilizing expensive oxides rather than a two component system used to make alumina-silica fibers.